1. Technical Field
The invention relates generally to nuclear fuel assemblies of the type having two end pieces or end nozzles joined together by tie rods along which are spaced apart grids for holding fuel rods in position and having an instrumentation tube one end of which is secured into a blind bore formed in one of said nozzles.
Assemblies of the above type are used in pressurized water reactors. An instrumentation tube, placed along the vertical axis of the fuel assembly, is adapted to receive a flexible tube, called a "glove finger", movable through the bottom of the reactor vessel so as to be inserted into the core during operation of the reactor and withdrawn from the core during fuel reloading. When a "glove finger" is in the core, a measuring probe may be inserted into the core and removed therefrom by a flexible line remotely controlled by an electromechanical assembly situated outside the reactor vessel.
2. Prior Art
In a typical fuel assembly, one of the ends of the instrumentation tube opens through a coolant flow restriction intended to limit the flow rate which circulates in the space between the "glove finger" and the instrumentation tube. Different considerations lead to giving the instrumentation tube an internal diameter considerably greater than the external diameter of the "glove finger" which is engaged therein. The resulting gap and the lack of guidance in the spaces between the different internal components of the reactor, cause vibrations of the "glove finger" subjected to the coolant flow along the instrumentation tube, possibly to transverse coolant streams and to the vibrations of the internal parts which support the guide elements.
In the power reactors now in service, the high coolant speed causes sufficiently intense vibrations to cause wear of the "glove fingers" and instrumentation tubes. Now, a breakage in the instrumentation tube may cause transverse coolant jets to appear toward the adjacent fuel rods in the fuel assembly.